In a known fuel injector, a piezoelectric actuator is operable to control the delivery of fuel into a combustion space. The piezoelectric actuator typically includes a stack of piezoelectric elements across which a voltage is applied in use through an electrical connector. A fuel injector of this type is described in the Applicant's granted patent EP 0995901.
It is known to arrange the piezoelectric stack within an accumulator volume in the injector that is arranged, in use, to receive high-pressure fuel. Commonly, therefore, the piezoelectric stack is immersed in fuel throughout its operational life. Moreover, when the fuel injection system is in use, the actuator is exposed to fuel at rail pressure ranging from, for example, 200 to 2000 bar. To protect the piezoelectric stack from damage, it is important that the stack is sealed off from fuel within the accumulator volume, in order to prevent the ingress of fuel into the joints between the individual elements forming the stack. It is also important to protect the piezoelectric stack from environmental contaminants such as moisture. The presence of a conductive fluid such as water in the stack can cause electrochemical effects and lead to a short circuit failure.
It is particularly important that when the actuator is used in a hydrostatically pressurized mode, as described for example in the Applicant's granted patent EP 1096137, the piezoelectric stack is protected from the ingress of hydrostatic pressurising fluid, as this could migrate into cracks in the structure and cause them to open up.
In EP 0995901, the piezoelectric stack is provided with a coating or sleeve composed of a flexible sealant material. The coating helps to seal the elements of the piezoelectric stack from fuel in the accumulator volume. Further, as the coating is flexible, the stack is subjected to the compressive load applied by the fuel under pressure, helping to reduce the propagation of cracks in the structure. For this purpose, it is known to use an over-moulding technique to encapsulate the piezoelectric stack within a plastics coating, or a sleeve member as described in the Applicant's granted patent application WO 02/061856. The coating or sleeve may also encapsulate the electrical connector, as described in the Applicant's granted patent EP 1079097. The sleeve is preferably formed from a fuel-resistant, low permeability fluoropolymer such as, for example, polyvinylidene fluoride (PVDF) or ethylene tetrafluoroethylene (ETFE).
It has been observed, however, that there may still be ingress of fluid into the interface between the actuator and the coating or sleeve. For example, the aforementioned sleeves are preferably formed from elastic or heat-shrink materials as tubes with open first and second ends such that the actuator can be inserted into the open tube. The tube is then allowed to contract elastically, or is made to contract by the application of heat, in order to encapsulate the actuator. It will be understood that although the sleeve now bears tightly against the actuator, the first and second ends of the tube present paths for the ingress of fluid into the interface between the coating and the encapsulated actuator.
It is known to provide a chemical adhesive or a filler material, as described for example in the Applicant's co-pending international application WO 02/061856, between the coating and the encapsulated actuator. However, even in this case the ingress of fluid between the coating and the actuator has been observed. The currently used adhesives are prone to degradation by fuel and ingress is usually observed after less than 2000 hours. It is reasonable to expect an injector to have a lifetime of 15 years (>130000 hours). A variety of different adhesives has been tried in an attempt to create durable end sealing, but it has been difficult to achieve consistent durable bonding.
End sealing has also been a problem with injection-moulded encapsulation. Increasing the end seal leak path with a labyrinth has met with varying degrees of success, but with the additional disadvantage that it cannot easily be used with ceramic components without extra cost being incurred. A further difficulty has been the tendency for piezoelectric elements to fracture during injection moulding.
It is an object of the present invention to alleviate the above mentioned problems but without adding significantly to the cost, complexity or size of the actuator.